The natural intervertebral disc contains a jelly-like nucleus pulposus surrounded by a fibrous annulus fibrosus. Under an axial load, the nucleus pulposus compresses and radially transfers that load to the annulus fibrosus. The laminated nature of the annulus fibrosus provides it with a high tensile strength and so allows it to expand radially in response to this transferred load.
In a healthy intervertebral disc, cells within the nucleus pulposus produce an extracellular matrix (ECM) containing a high percentage of proteoglycans. These proteoglycans contained sulfated functional groups that retain water, thereby providing the nucleus pulposus within its cushioning qualities. These nucleus pulposus cells may also secrete small amounts of cytokines as well as matrix metalloproteinases (“MMPs”). These cytokines and MMPs help regulate the metabolism of the nucleus pulposus cells.
In some instances of disc degeneration disease (DDD), gradual degeneration of the intervetebral disc is caused by mechanical instabilities in other portions of the spine. In these instances, increased loads and pressures on the nucleus pulposus cause the cells to emit larger than normal amounts of the above-mentioned cytokines. In other instances of DDD, genetic factors, such as programmed cell death, or apoptosis can also cause the cells within the nucleus pulposus to emit toxic amounts of these cytokines and MMPs. In some instances, the pumping action of the disc may malfunction (due to, for example, a decrease in the proteoglycan concentration within the nucleus pulposus), thereby retarding the flow of nutrients into the disc as well as the flow of waste products out of the disc. This reduced capacity to eliminate waste may result in the accumulation of high levels of toxins.
As DDD progresses, the toxic levels of the cytokines present in the nucleus pulposus begin to degrade the extracellular matrix (in particular, the MMPs (under mediation by the cytokines) begin cleaving the water-retaining portions of the proteoglycans, thereby reducing its water-retaining capabilities). This degradation leads to a less flexible nucleus pulposus, and so changes the load pattern within the disc, thereby possibly causing delamination of the annulus fibrosus. These changes cause more mechanical instability, thereby causing the cells to emit even more cytokines, thereby upregulating MMPs. As this destructive cascade continues and DDD further progresses, the disc begins to bulge (“a herniated disc”), and then ultimately ruptures, causing the nucleus pulposus to contact the spinal cord and produce pain.
Intervertebral disc degeneration causes a number of clinical problems, including sequelae related to reduced disc height and herniation. In many cases, a simple discectomy can effectively relieve pain, but in time results in further collapse of the disc space because the intervertebral disc can no longer resist body loads the same as a healthy disc. Spine fusion procedures represent another state of the art treatment for disc problems. Fusion generally involves the use of interbody fusion cages and spinal fixation systems to immobilize the fusion site.
In an effort to substantially maintain the patient's range of motion and to reduce tissue damage associated with surgical intervention, the art has considered nucleus pulposus replacement and enhancement devices. Many of these devices are designed to fill at least a portion of the void left by removal of the nucleus pulposus portion of the disc and provide joint flexibility and shock absorption. Some of the nucleus pulposus devices being evaluated are in situ cured (such as in situ cured polyurethane contained within an outer bladder and in situ cured protein polymers). Other devices under evaluation include relatively solid hydrogels (such as hydrogel contained within a UHMWPE pillow and hydrogel balls).
Other intervertebral motion devices include devices having an articulation interface and cushion-type devices.
Both the fusion and motion intradiscal implants require an accurate determination of the cleared disc space for the best performance, mechanical fit and material interdigitation of the device in order to minimize potential device movement and expulsion. Each of the above-noted treatments involving an implant requires a removal of the natural nucleus pulposus from the disc space. This procedure is called a “discectomy”.
The ability of a surgeon to accurately determine the position, size and shape of the cleared disc space during discectomy is currently limited by many factors, including the procedure approach, access, location and the size through the annular wall, as well as available intraoperative imaging techniques. Improper location, size or shape of the cleared disc space following discectomy can greatly impact the size, placement and securement of intervertebral devices that are ultimately placed in the disc space, as well as the biomechanical loading of the device and the physiologic response to the device. For example, improper lateral placement of a nucleus pulposus replacement device may cause migration or expulsion of this implant, leading to continued height loss and irritation of neighboring tissues (including nerve roots), thereby creating additional pain or requiring re-operation.
When attempting to replace the nucleus of a damaged disc with a nucleus replacement implant, the surgeon typically desires to attain a number of related goals. First, the surgeon has a desire to adequately fill the disc space following the removal of disc tissue, while avoiding unnecessary damage to the surrounding annulus fibrosus. There is a further desire to intraoperatively visualize the space to be occupied by the nucleus replacement so that the implant may be effectively implanted. Next, there is a desire to avoid expulsation of the implanted device, which may occur either through the port through which the implant is inserted or through natural annular fissures. Lastly, there is a recognition that addition of radio-opaque agents to the implant may have a detrimental effect upon the performance properties of the implant.
It is known in the medical field to deliver a curable material to a surgical site within an expandable device or membrane, such as a balloon. In some embodiments thereof, a curable cement is delivered to a fractured vertebral body in order to strengthen the structure a regain its stability. In the area of nucleus pulposus replacement, it is known to deliver the curable material through a catheter.
For example, US Published Patent Application Number 2005/0027358 (“Suddaby”) discloses a nucleus replacement including a distendable sack or balloon which is inflated with a hardenable material and is detached in situ when the injected material has hardened. Suddaby further teaches that two nested balloons may be inserted, and then filled with materials which have different hardnesses when cured, to simulate a natural disc.
US Published Patent Application Number 2005/0245938 (“Kochan”) discloses repair of intervertebral discs with a catheter for inserting through a cannula, the catheter having a distal end and a proximal end and a lumen extending longitudinally therethrough. An expandable balloon may optionally be detachably attached to the catheter near the distal end. The proximal end of the catheter is coupled to an injector that holds a supply of a thermoplastic elastomer material at a predetermined elevated temperature sufficiently high to maintain the thermoplastic elastomer at a liquid state. The device allows a thermoplastic elastomer material to be injected into the intervertebral disc space or the articular joint space as a replacement prosthetic for the disc's nucleus pulposus.
US Published Patent Application Number 2005/0251259 discloses a system for replacing a natural vertebral disc with a synthetic disc, said system comprising an outer balloon adapted to be inserted into an intervertebral disc space, an inner balloon which can be inserted within said first balloon, thus defining a chamber between said first balloon and said second balloon, a first hardenable material in liquid form adapted to be injected into said inner balloon, a second hardenable material in liquid form adapted to be injected into said chamber, said first and second materials having different properties when hardened, and means for injecting said materials into said respective balloons while they are disposed within said intervertebral disc space, whereby a synthetic disc having inner and outer portions with different properties can be formed in said intervertebral space.
US Published Patent Application Number 2005/0209602 discloses an apparatus adapted to deliver a flowable biomaterial to an intervertebral disc space, comprising: a reservoir containing the flowable biomaterial fluidly coupled to the intervertebral disc space; at least one sensor adapted to monitor at least one injection condition of the flowable biomaterial; a controller programmed to; monitor the at least one sensor; control the flow of the flowable biomaterial into the intervertebral disc space in accordance with a first operating parameter; controlling the flow of the flowable biomaterial in accordance with a second operating parameter in response to one or more of the injection conditions reaching a threshold level; and maintaining the second operating parameter during at least a portion of the curing of the flowable biomaterial. incorporates sensors into its system.
US Published Patent Application Number 2003/0195628 discloses a method for repairing a damaged or diseased intervertebral disc, the method comprising the steps of: using minimally invasive techniques to remove damaged or diseased nucleus from the disc; providing a mold apparatus comprising a balloon adapted to contain a biomaterial and a delivery cannula adapted to flowably connect a biomaterial source to the balloon; positioning the balloon in the intervertebral disc space using minimally invasive techniques; providing a biomaterial source comprising a plurality of components adapted to be mixed at the time of use to provide a flowable biomaterial and initiate its cure; mixing the biomaterial components; delivering the flowable biomaterial into the balloon using minimally invasive techniques to provide a distraction pressure to the intervertebral disc space; allowing the delivered biomaterial to cure to permit the cannula to be removed and to provide a permanent replacement for the nucleus; and applying mechanical distraction in combination with the pressurized injection of flowable biomaterial to distract the intervertebral disc space.
US Published Patent Application Number 2005/0113923 teaches a method for implanting a spinal disc nucleus pulposus implant, comprising: removing nucleus pulposus tissue from a spinal disc; and injecting a biocompatible material into an intradiscal space; wherein the biocompatible material is injectable into the intradiscal space in a fluid state below physiological temperatures, and is curable by temperature alone via a reversible phase shift to form a gel at physiological temperatures.
US Published Patent Application Number 2005/0065609 discloses a flexible prosthetic cover shaped to form a replacement nucleus pulposus for an intervertebral disc and comprising an aperture for the introduction of filling material therein, and an elongate introducer member configured to pass into the aperture, the cover having a strengthened portion substantially opposite the aperture for engaging the distal end of the member, the strengthened portion and the said distal end being arranged to interlock, for facilitating orientation of the cover.
U.S. Pat. No. 5,888,220 (“Felt I”) discloses a nucleus pulposus replacement device comprising an expandable bag into which in-situ curable polyurethane is injected. Felt further discloses that the placement of the bag can be radiographically verified with the use of a C-arm. See also U.S. Pat. No. 6,248,131, US Published Patent Application Nos. US 2003/0220649 (“Felt II”) and US 2003/0195628. Felt II discloses some embodiments in which the balloon has metallic wires or other imageable means incorporated into it so that the balloon can be seen under fluoroscopy. Felt discloses that potential imageable materials include any metal, metal alloys, or ceramics that could be combined with a polymer, and that the imageable material can be in the form of wires, a mesh, or particles incorporated into the balloon or on its surface.
Felt does not disclose the use of a radiographic disc space trial balloon that is inflated to verify the size and geometry of the disc space.